1. Field of the Invention
The present disclosure relates to a display device. Especially, the present disclosure relates to an organic electroluminescent display device.
2. Discussion of the Related Art
For satisfying the information society developed by the information communication technology, the demands for the electric displays are increasing and the kinds of the display required are varied. To satisfy these requirements of the modern information society, the display element is required to be more precise, larger, thinner, and smallized and to be manufactured with lower cost. To do so, the conventional cathode ray tube (or “CRT”) is replaced with new flat panel display (or “FPD”).
Especially, the development for the devices relating to the communication and computer such as semiconductor and display is important for advancing the technology in each field. Among the various display elements currently used, the organic electroluminescent display device is one display type occupying the attention for the color thin flat panel display. As the organic electroluminescent display device has excellent characteristics for the graphic display, the television display and the flat panel display such as wide viewing angle, speed response, and high contrast. Further, as it has thin thickness, light weight and good color representation, it is suitable for the next generation flat panel display.
The organic layer of the organic electro luminescent display device comprises the hole injection layer (or “H1L”), the hole transport layer (or “HTL”), the emission layer (or “EML”), the electron transport layer (or “ETL”) and the electron injection layer (or “EIL”). Any or some of these elements consisting of the organic layer excepting the emission layer may be differently composed in the organic layer according to the material or the structure of the organic luminescent display device. The organic electroluminescent display device emits the light by forming exciton when the hole generated at the anode electrode and the electron generated at the cathode electrode are jointed at the emission layer after they are passing through the hole injection layer and the electron injection layer, respectively.
For the ideal cases, the hole and the electron should meet at the middle portion of the emission layer. However, due to the materials, the characterisitics, and the thickness of the above mentioned layers, the difference between the mobilities of the hole and electron, and the energy level difference of the each layer (“HOMO” or “LUMO”), the hole and the electron cannot meet at the middle portion of the emission layer, in actual conditions.
Generally, the hole mobility in the organic material used for the hole transport layer can be 10˜100 times faster than the electron mobility in the organic material used for the electron transport layer. Therefore, the holes may be earlier transported or injected into the organic emission layer than the electrons. In the interim, in the same organic material, the hole mobility and the electron mobility are decided according to the structure of the organic material. For example, as shown in FIG. 6a, when using an organic material in which the hole mobility is faster than the electron mobility as the emission layer 178, the electrons and holes are colliding near the interface between the electron transport layer 174 and the emission layer 178, not the middle portion of the emission layer 178. In this case, in addition, the difference of HOMO (“highest occupied molecular orbital”) between the emission layer 178 and the electron transport layer 174 is very small due to the material characteristics, so that the hole can move to the electron transport layer 174 easily. As a result, the function of the electron transport layer 174 may be degraded and then it may cause the reduction of the life time of the organic electroluminescent element.
In order to overcome the above mentioned drawback, the organic layer may comprise the emission layer only or the material characteristics of each layer may be improved. However, these solutions are seriously depended on the materials, as well as it is hard to find exactly suitable material for the characteristics of the display device. Therefore, there are a lot of researches for enhancing the quality of the organic electroluminescent display device by improving the structure of the device.